Production facilities for making asphalt concrete to be used as a paving composition are well-known. Generally, such production facilities can be categorized as either batch plants or continuous production plants. In a conventional batch plant, a quantity of aggregate materials is heated and dried and placed in a mixer along with a quantity of liquid asphalt cement. The aggregate materials and asphalt cement are thoroughly mixed and discharged as a batch of asphalt concrete into a storage bin or a delivery truck. In a conventional continuous production plant, aggregate materials and asphalt cement are continuously introduced into the plant and asphalt concrete is continuously being produced. Since the raw materials for asphalt concrete are continuously being introduced, the proportions of the components in the mix must be controlled by metering the relative rates at which the various components are introduced.
A common type of continuous production plant is a combination dryer/mixer. In one such device, aggregate materials are introduced into the upper end of an inclined rotating drum. A burner mounted at the upper end of the drum heats the air flowing through the drum, and the aggregate material is heated and dried as it is tumbled through the heated gas flow in the drum. Liquid asphalt cement is introduced into the lower part of the drum, where it mixes with the dried aggregate materials to produce an asphalt concrete mixture. This type of dryer/mixer is known as a parallel-flow mixer, because all of the materials move through the drum in a direction away from the source of the burner flame. One disadvantage of a parallel-flow mixer is that it is not thermally efficient. Aggregate materials are generally wet and cold when introduced into the mixer. Before such materials can acquire significant heat, the moisture they hold must be evaporated. In a parallel-flow mixer, the aggregate materials are exposed to the highest temperatures while still cold and damp. By the time the aggregate materials are dried, they have moved down the drum into a cooler zone.
U.S. Pat. No. 4,867,572 of Brock et al. describes a type of continuous production plant known as a counter-flow dryer/mixer. This mixer comprises an inclined inner drum mounted for rotation about its long axis. An outer drum is disposed around the rotating inner drum so as to form an annular space between the outside of the inner drum and the inside of the outer drum. Flights or paddles are mounted on both the inner and outer surfaces of the inner drum. A burner is located at the lower end of the inner drum, and aggregate materials are introduced into the upper end of the inner drum. Because of the inclination and rotation of the inner drum, aggregate materials that are introduced into the upper end of the inner drum are dried and heated as they are tumbled down towards the lower end and towards the source of the burner flame. At the lower end of the inner drum, the dried and heated aggregate materials are discharged into the annular space between the inner drum and the outer drum. Liquid asphalt cement is also introduced into this annular space, and continued rotation of the inner drum causes the asphalt cement to be thoroughly mixed with the heated and dried aggregate materials to produce an asphalt concrete mixture. The flights on the outside of the rotating inner drum assist in this mixing and also serve to direct the asphalt concrete mixture towards the upper end of the inner drum, as the inner drum is rotated, to an asphalt concrete discharge outlet.
Asphalt concrete is also conventionally made in continuous plants comprised of separate dryers and mixers. Some such plants employ a rotating dryer drum in which aggregate materials are introduced. A burner is located at one end of the drum and the aggregate materials are moved along the drum through the heated gases generated by the burner in either parallel flow or counter-current flow to an outlet. A separate mixer, such as a rotating drum mixer or a pugmill, is employed to mix the dried aggregate materials from the dryer drum with liquid asphalt cement.
Because some conventional mixers (including dryer/mixers) expose liquid asphalt cement to the high-temperature gases used for drying and heating the aggregate materials and to the steam generated in the drying process, a “blue smoke” of hydrocarbon gases can be stripped from the light oil components of the asphalt cement. Although relatively insignificant as an emission (by weight), “blue smoke” is visible and can be unsightly. In order to eliminate “blue smoke”, it has been deemed desirable to (1) direct the “blue smoke” into the burner for incineration, or (2) filter the “blue smoke” from the plant and condense it for disposal. U.S. Pat. No. 5,054,931 of Farnham et al. describes a counter-flow drum mixer in which a burner shield is employed to isolate the liquid asphalt cement from the hot gases generated by the burner. A venturi mounted in the shield directs all fumes, including steam and “blue smoke”, produced in the lower end of the drum through the flame of the burner. In a second embodiment, the Farnham patent describes the creation of a foamed asphalt cement mixture by mixing liquid asphalt cement with water and pressurized air. This foamed mixture may then be introduced through a conventional asphalt cement inlet pipe (instead of liquid asphalt cement) into the area between the burner and the burner shield to be mixed with the dried aggregate materials therein.
U.S. Pat. No. 6,846,354 of Larsen et al. describes a method for making asphalt concrete using two types of asphalt cement, a hard component and a soft component. According to this method, the hard component comprises an asphalt cement foam having a penetration of less than 100 dmm, whereas the soft component comprises asphalt cement having a penetration of at least 200 dmm. According to the method of the Larsen patent, a quantity of aggregate material is heated to a temperature of approximately 265° F. (130° C.). This heated aggregate material is then mixed with a quantity of soft asphalt cement having a temperature of about 247° F. (120° C.) to form a warm mix of asphalt concrete. A hard asphalt foam is then made by injecting a hard component of asphalt cement into a foaming chamber, injecting water into the asphalt cement flow, and then homogenizing the foam mixture in a static mixer in a mixing chamber. A quantity of this hard asphalt cement foam equal to that of the soft asphalt cement previously added is then dispensed from the mixing chamber into the warm mix and mixed therewith. A filler comprising about 5% of the end product is added to this mixture at about 67° F. (20° C.) to produce the end product.
It is believed that the Larson process requires a foamed hard component of asphalt cement to reduce the temperature of the resulting asphalt concrete mix and a soft component to insure sufficient compaction of the resulting product. It would be desirable if a method and apparatus could be developed which is capable of employing a single grade of asphalt cement to produce an asphalt concrete product at temperatures lower than conventional hot mix temperatures. It would also be desirable if such a method and apparatus could be developed that would obtain sufficient compaction without requiring inclusion of a soft component of asphalt cement.